"Why" is a difficult question for science to tackle. How do cells age? That question can get you loads of information. When and where do the changes associated with aging occur? Again, there is plenty of literature addressing that. What happens to cells as they age? Sure, again that is a common research topic and plenty has been done. "Why", however, is more a question for philosophers than for science -- it's not very testable (technically, hypotheses about "why" are often not falsifiable).

Try some keywords using the term "senescence" in Wikipedia, PubMed, etc. and you'll find entries into the topic of aging. Generally though they won't answer the "why" question.

There are various factors that contribute to aging of a cell. I don't think you'll find a single switch that explains it all. An interesting topic in aging is the disease Hutchinson-Gilford progeria, which ties in with the Lamin A topic I mentioned above.

Telomeres are repeating (and I'm pretty sure, noncoding) sequences of DNA at the tips of chromosomes. As the cell repeatedly replicates itself, bits of the ends of the chromosome don't copy, so the telomere part is giving the chromosomes leeway. Eventually, the telomere shortenings accumulate and will cut into actual coding DNA, leading to signs of aging. (Though I looked it up to double-check myself, and websites said that the bits that don't copy don't cut into the coding DNA—once it reaches the coding DNA, it makes the cell stop dividing. That sounds strange; I like what I learned, better.. hah)

As everybody said, look up for Telomeres!Telomeres are sequences of the DNA that are added at the tips of chromosomes, and as a cell is passing through its division cycle, the DNA is replicated and the ends are shortened as a result of the END REPLICATION PROBLEM (read about this one, it will be useful to understand it). As these parts are shortened, at some point the ends will be almost near the coding DNA leading the cell to undergo apoptosis before the coding ends are cut as well!

Telomerase is an enzyme that adds telomeres to the ends of chromosomes. The problem is that telomerase exists only in the cancerous cells (and I don't actually understand why) and germ cells. Therefore, because it doesn't exist in the normal somatic cells, these cells will die at some point, and well, as you know, not all body's tissues can regenerate, and so those which die will progress to show some aging symptoms. Take a skeletal muscle tissue for example, it doesn't regenerate and as its cells die by aging, muscle weakness will result!

citroenboom wrote:And what about my precious E. coli! They do not contain telomers

They age because of damage by the environment, by dividing, DNA damage and that sort of things.

In Groningen University (The Netherlands) is some nice research going on about this topic (as I understood)

Interesting thought, many years ago there were a few experiment about the regenerative abilities of bacteria cells. Basically they intentioned through heat, chemical, radiation etc etc induced damages to a few eColi colonies. examined them, and tagged a few cells/nucleotides, and cultured them for a few gens. Result, the later bacteria went on living normally. colonies grew.